If you are using an on submit page process then you are able to enter a success or fail message to display once the process is complete and the page reloads, see: 8.5 Understanding Page Processes
To display the message you will need on-submit after page processing branch that has the "include process success message" box checked.
If you are not using this method then you would need describe how the update happens.
Hope this helps.
I'm using a dynamic action on a custom button, not submit.
It's an execute PL/SQL code action, which calls a procedure.
I guess you are updating using DA.
You can do one thing here.
Create a DA
Execute your PL/SQL Code and set success message to one of the page items and return that value from the PL/SQL Code you just executed
I created a new item that resets before the update and sets a message after it.